The present invention relates to braking systems for electric vehicles or hybrid electric vehicles, and in particular to braking systems which comprise a regenerative braking system in conjunction with a conventional, mechanical friction braking system. As the term is used herein, "electric" vehicles are those which are propelled solely by an electric motor which receives energy from an on-board storage battery; "hybrid electric" vehicles are those which may be propelled, generally at different times, by either an electric motor, which receives energy from either an on-board storage battery or from a generator operatively coupled to an internal combustion engine (a "series hybrid electric vehicle"), or by an internal combustion engine selectively operatively coupled to the ground-engaging wheels (a "parallel hybrid electric vehicle"). Thus, while both electric and hybrid electric vehicles comprise a rechargeable on-board storage battery, electric vehicles are propelled by an electric motor alone, whereas hybrid electric vehicles are propelled by an electric motor and also comprise an internal combustion engine which is also used for propulsion or, alternatively, for generating electrical power via a generator to recharge the battery.
Generally, electrical power is connected to the motor only when driving propulsion is demanded by the operator (e.g., by pressing on the accelerator or "gas" pedal). At other times (during coasting or braking) the power feeding the motor is disconnected. The inertia of the moving vehicle, however, continues to rotate the rotor of the motor, which is coupled to a ground-engaging wheel of the vehicle. Regenerative braking systems use the rotating motor as a generator which works against an electrical load placed in communication with the motor/generator upon actuation of a vehicle braking control device, such as a brake pedal. The electrical load comprises the partially-depleted battery, which is at least partially recharged by the motor acting as a generator powered by the still-moving vehicle's inertia. The electrical load on the motor/generator slows the rotational speed thereof, thereby braking the vehicle.
Previous electric or hybrid electric vehicles have employed hybrid brake systems comprising both an electrical, regenerative brake system and a conventional, mechanical friction braking system. Generally, such vehicles couple the two brake systems so that they work together, with the regenerative braking system being first actuated upon initial depression of the brake pedal to slow the vehicle and provide a charge to the battery. Further depression of the brake pedal then additionally actuates, or completely switches to, the conventional mechanical braking system for stopping the vehicle. A problem encountered with prior hybrid brake systems, however, is that the transition from only regenerative braking to mechanical braking, alone or in combination with regenerative braking, has been abrupt, resulting in poor brake pedal "feel" and impairing the driveability the vehicle.
For example, some prior vehicles employing hybrid braking systems used only regenerative braking until the brake pedal was depressed a considerable distance toward the floorboard. Drivers operating these vehicles felt very little pedal resistance and encountered sluggish transition from regenerative braking to conventional braking because there was little or no pressure in the brake lines. Drivers of these vehicles overcorrected by stomping on the brakes, abruptly applying the conventional mechanical brake system, thereby causing erratic handling of the vehicle.
A means for providing a smooth transition between regenerative and conventional braking in an electric or hybrid electric vehicle is desired.